A new Carrier Type (NCT) has been introduced along with the constant evolvement of the Long Term Evolution-Advanced (LTV-A) standard, where there is no legacy Physical Downlink Control Channel (PDCCH) in the new NCT, and also a common search space of Enhanced PDCCHs (EPDCCHs) needs to be introduced considering that the new NCT may operate as a separate carrier.
An EPDCCH User Equipment (UE or referred to as a terminal)-specific search space has been introduced to the LTE Rel-11, and for a Physical Resource Block (PRB) pair in which EPDCCHs are transmitted, the following categories of Resource Elements (REs) will be precluded from REs in which EPDCCHs can be transmitted:
EPDCCHs can not be transmitted in REs for cell-specific Reference Signals (RS's), or REs for cell-specific RS's at Quasi Co-Locations (QCLs).
EPDCCHs can not be transmitted in REs configured for Zero Power/Non-Zero Power Channel State Information-Reference Signals (ZP/NZP CSI-RS's), and these REs can be obtained from UE-specific Radio Resource Control (RRC) signaling.
EPDCCHs can not be transmitted in REs in Orthogonal Frequency Division Multiplexing (OFDM) symbols before an EPDCCH starting symbol lEPDCCHStart, which can be obtained by detecting a Physical Control Format Indicator Channel (PCFICH), or which can be configured via higher-layer signaling.
The ZP/NZP CSI-RS's will be introduced below:
FIG. 1 illustrates a configuration diagram of CSI-RS's in the LTE system of Rel-11, where the sub-frame structure type 1 and 2 in FIG. 1 represent general CSI-RS patterns. In FIG. 1, FIG. 1(a) illustrates the multiplexing scheme with 2 ports, FIG. 1(b) illustrates the multiplexing scheme with 4 ports, and FIG. 1(c) illustrates the multiplexing scheme with 8 ports. Each UE is configured separately with the number of CSI-RS ports, and the positions of CSI-RS resources so that different resource positions may be occupied by different UEs. Moreover each UE can be further configured with resources of zero-power CSI-RS's configured as 4-port CSI-RS's, where no signals will be transmitted over the resources of the zero-power CSI-RS's, and the zero-power CSI-RS's are configured via RRC signaling. If certain 4-port CSI-RS pattern is configured for zero-power CSI-RS's, then the UE may determine that no PDSCH data will be transmitted at the positions of REs.
Since the ZP/NZP CSI-RS's are configured per UE and periodically, if a UE residing over an NCT carrier is detecting an EPDCCH Common Search Space (CSS), then the UE can not know the configuration of the ZP/NZP CSI-RS's in Physical Resource Block (PRB) pairs in a current sub-frame. As illustrated in FIG. 1, a box which is not filled represents a PDSCH data area, and if only REs in unfilled boxes are occupied by EPDCCH CSS's, then resources may be wasted.
Table 1 depicts the CSI-RS configurations in the 36.211 protocol, where the first 20 configurations are applicable to both of the sub-frame structure type 1 and 2, and the last 12 configurations are applicable to only the sub-frame structure type 2. FIG. 2(a), FIG. 2(b) and FIG. 2(c) illustrates additional 2, 4 and 8-port CSI-RS's supported in the sub-frame structure type 2 respectively.
TABLE 1Mapping from CSI reference signal configurationsto (k′, l′) for a normal cyclic prefixCSI referenceNumber of configured CSI reference signalssignal1 or 248configuration(k′, l′)ns mod 2(k′, l′)ns mod 2(k′, l′)ns mod 2Frame0(9, 5)0(9, 5)0(9, 5)0structure1(11, 2) 1(11, 2) 1(11, 2) 1type 1 and 22(9, 2)1(9, 2)1(9, 2)13(7, 2)1(7, 2)1(7, 2)14(9, 5)1(9, 5)1(9, 5)15(8, 5)0(8, 5)06(10, 2) 1(10, 2) 17(8, 2)1(8, 2)18(6, 2)1(6, 2)19(8, 5)1(8, 5)110(3, 5)011(2, 5)012(5, 2)113(4, 2)114(3, 2)115(2, 2)116(1, 2)117(0, 2)118(3, 5)119(2, 5)1Frame20(11, 1) 1(11, 1) 1(11, 1) 1structure21(9, 1)1(9, 1)1(9, 1)1type 2 only22(7, 1)1(7, 1)1(7, 1)123(10, 1) 1(10, 1) 124(8, 1)1(8, 1)125(6, 1)1(6, 1)126(5, 1)127(4, 1)128(3, 1)129(2, 1)130(1, 1)131(0, 1)1
Since the UE detecting the EPDCCH CSS can not know the configuration information of the ZP/NZP CSI-RS's in the current sub-frame in the NCT, the UE can not determine REs in which EPDCCHs can be transmitted, in the PRB pairs of the current EPDCCH CSS, and consequently can not map the REs which are used for EPDCCH CSS transmission.
Intuitively a solution is to reserve all the REs which may be configured for ZP/NZP CSI-RS's, so that the EPDCCH CSS will not be mapped onto these reserved REs but will be mapped onto only the other available REs, and although this solution can address the problem above, there may be a considerable overhead because some REs which may be configured for ZP/NZP CSI-RS's are not configured for ZP/NZP CSI-RS's, though the EPDCCH CSS can be transmitted also in these REs.